The circuit design for controlling voltage/current utilizing high-side FET and low-side FET have many applications involving regulation of electrical power supplies of the integrated circuit. In such application, current flows from the junction between the source of high-side FET and the drain of low-side FET to the load. This load is connected in series with an inductor and connected in parallel with a capacitor. When the cycle of operation starts, high-side FET is turned on, low-side FET is turned off allowing current to flow through high-side FET to inductors, capacitors and load. This current increases as the capacitor charges. When the voltage across the load reaches the target level, the high-side FET is turned off, low-side FET is turned on, and this current decreases as the capacitor discharges. Hence by switching the high side FET and low side FET alternatively between on and off, the output voltage will not be changed due to the increase or decrease of the inductor current.
As shown in FIG. 1, the conventional switching controller consists of a comparator 10, a high-side FET 12 and a low-side FET 14. The drain of the high-side FET 12 is electrically connected to the input voltage VIN, and the source of the high-side FET 12 is connected to the drain of the low-side FET 14. The source of the low-side FET 14 is connected to ground. The gate of the high-side FET 12 and that of the low-side FET 14 are respectively connected to the comparator 10. When a sufficient voltage is applied to the transistor gate electrode, corresponding current will flow between the drain and the source of the transistor. Through voltage/current control operation, the gate of high-side FET 12 and that of low-side FET 14 are alternately switched on and off. Furthermore, an inductor 16 is connected to the junction connecting the source of high-side FET 12 and the drain of the low-side FET 14. A load 18 is connected in series with the inductor and to the ground, and the voltage across the load 18 is the output voltage Vo.
When the high-side FET and the low-side FET is about to switch from on to off or vice versa, the gate of the transistors will remain in the original state of on or off for some time. As such, a feedback voltage F is generated from the output voltage VO using a voltage divider 20. This feedback voltage F is fed to the comparator 10 and is compared to the target voltage T produced by a target voltage generator 22. When the feedback voltage F is equal to the target voltage T, the comparator 10 produces trigger signal to switch the on/off state of the high-side FET 12 and low-side FET 14 respectively. In addition, a capacitor 24 is connected in parallel with the load 18. The capacitor delays the output voltage across the load 18 relative to the output current through the load 18. This leads to an unstable output voltage VO.
As mentioned above, the conventional switching controllers utilize the ripple in the output voltage Vo to regulate the output current. However, this ripple is undesirable in certain circuit applications. Therefore equivalent series resistance (ESR) 26 of the capacitor 24 is used to control the increase and decrease of the output voltage Vo. To achieve this purpose, the equivalent series resistance 26 has to be large enough so that the capacitor 24 acts like a resistor, so the phase of the output voltage Vo across load 18 corresponds to the leading current flowing through the load 18, thereby enabling stable operation.
For example, a ceramic capacitor with an ESR of 20 million ohms may be replaced with a tantalum capacitor with an ERS of 200-600 million ohms. However, the calculation may be difficult for the engineer since the comparator 10 and FETs 12, 14 are manufactured and packaged by the first manufacturer while load 18 and capacitor 24 are made by second manufacturer; thus the ESR 26 is usually out of control of the first manufacturer. Furthermore, the ESR 26 of the capacitor 24 often depends on factors such as the manufacturing method, material, and temperature of the capacitor.
Accordingly, in view of the above problems, the present invention proposes a regulating method to solve the problems arising from conventional design.